The present invention relates generally to pneumatic tires, and more particularly to pneumatic tires equipped with a specially designed tread to reduce the amount of noise generated by the tire as it rolls upon a given surface.
Among the many factors which relates to the generation of noise by pneumatic tires, there are four factors which are most paramount. These factors are (1) resonant lug vibration, (2) slip/stick vibration, (3) air pumping and (4) resonant reinforcement.
"Lug vibration" results from stressing the lugs in the contact patch (the footprint or interface between the road and the tread) in mutually perpendicular directions within the plane of the contact patch as well as in a direction normally of the contact patch. The fore and aft stresses of the lugs in the contact patch play the greatest roll in generating noise. As each lug leaves the contact patch, the stresses are suddenly relieved and the lug thereby "pops out" of the contact patch, undergoes severe vibrations and generates noise.
"Slip/stick vibration" occurs as various portions of the tread at least partially slide ("slip") over the road in different directions at various spots in the contact patch. In the course of such sliding, at one or more other spots in the contact patch the horizontal shear stresses are usually sufficiently low to allow interfacial (contact patch) friction to instantaneously "anchor" one or more portions (for example, one or more lugs) of the tread to the road at the spots of low stress level. Continued partial sliding of the tread, however, results in a build-up of stress in the "anchored" lugs which releases them from their anchored condition and causes them to undergo an instantaneous slip.
If the tire is one which is prone to permit the vibration of its lugs to resonate, the "slipping" and "sticking" of the tread lugs, when in synchronism with the vibratory resonances of the tire, are subject to reinforcement causing the lugs to undergo more severe vibrations and generate noise of increased intensity.
"Air pumping" is the action of forcing air in and out of the voids and sipes in and between the tread lugs. As a given void or sipe enters the contact patch, its volume is suddenly compressed thereby expelling or pumping air out. As each void or sipe leaves the contact patch, its volume suddenly expands, thereby pumping air back in. This rhythic pumping of air in and out of the voids or sipes generates continuous pressure waves or sound energy and, if excessive, noise.
"Resonant reinforcement" involves certain mass distributions and elastomeric properties of a tire which result in the vibration of parts of the tire that respond to vibration-inducing energy imparted to the tire in a repetitively timed sequence by reacting to create sharply increased vibrations at various speeds of the tire. If the tread lugs are spaced from one another such that their excitation peak frequencies coincide with the resonant frequencies of the tire, resonant reinforcement is said to occur with the result of sound build-up.